1. Field of the Invention
The present invention relates to an inner circle cutting control apparatus for a numerically controlled machine tool which is supplied with commands relative to parameters including an initially cut radius I, a finally cut radius J, a radial depth of cut K, and a cutter diameter D, for linearly moving a cutter from the center of an inner circle in a workpiece by a predetermined distance according to the initially cut radius I, and then controlling the cutter to cut an inner circle in the workpiece according to the radial depth of cut K until the finally cut radius J is reached.
2. Description of the Prior Art
Numerically controlled machine tools are used in a wide variety of fields of art. Many applications require inner circles to be cut in workpieces by numerically controlled machine tools. For cutting an inner circle in a workpiece with a numerically controlled machine tool, it has been customary to give command parameters relative to the inner circle to be cut desired numerical values for the inner circle, and to control a cutter to machine the workpiece based on the parameter commands under numerical control.
FIG. 7 of the accompanying drawings shows a conventional inner circle cutting apparatus for numerically controlled machine tools. A desired inner circle is to be cut in a workpiece 4 by a cutter 2. The inner circle cutting apparatus is supplied with command parameters including an initially cut radius I, a finally cut radius J, a radial depth of cut K, a cutter diameter D, a cutting speed F, a final vertical position Z in an Z-axis direction, and a vertical depth of cut Q.
First, the cutter 2 linearly moves from the center of an inner circle in the workpiece 4 as indicated by the arrow 1 in a mode known as linear approach, and then cuts the workpiece 4 arcuately as indicated by the arrow 2 until the initial radius I is reached. Thereafter, the cutter 2 cuts the workpiece 4 along a full circumference according to the initial radius as indicated by the arrow 3, and then moves arcuately as indicated by the arrow 4, after which the cutter 2 linearly moves back to the center as indicated by the arrow 5.
The cutter 2 makes the above movement in one cutting cycle. In a next cutting cycle, the initially cut radius I is increased a certain interval based on the radius depth of cut K, and the above cutting cycle is repeated. The workpiece 4 is cut by the cutter 2 in successive cutting cycles until the radius of the cut circle reaches the finally cut radius J. The cutter 2 starts cutting the workpiece 4 in a vertical or Z-axis direction at a point Z0.
The linear movement of the cutter 2 from the center of the inner circle as indicated by the arrow 1, i.e., the linear approach mode, will be described in detail below with reference to FIG. 8 of the accompanying drawings. In the linear approach mode, when the initial radius I is given, the cutter 2 is linearly moved at 45.degree. by a distance of Icos45.degree.. At this time, the cutter 2 may be moved at a quick feed rate or a cutting feed rate. Designated at D is the diameter of the cutter 2.
When the cutter 2 cuts the workpiece 4 arcuately as indicated by the arrow 2, the cutter 2 moves arcuately through 90.degree. at a radius of 1/2I after it has been linearly moved in the linear approach mode.
If the cutter 2 moves at a quick feed rate in the linear approach mode, then the time required to effect the entire cutting process is shorter than if it moves at a cutting rate. However, in the event that the cutter 2 physically interferes with the workpiece 4 in the linear approach mode while the cutter 2 is moving at a quick feed rate, the cutter 2 or the machine tool supporting the cutter 2 is subjected to large stresses and the service life thereof is shortened.
FIGS. 9 and 10 of the accompanying drawings show the manner in which the path of the cutter 2 physically interferes with the workpiece 4. In FIG. 9, the path of the cutter 2 when it linearly moves in the linear approach mode physically interferes with the workpiece 4 at a point A. In FIG. 10, the radius of the inner circle is increased by the radial depth of cut K, and the workpiece 4 is to be cut by the cutter 2 according to the increased radius. The workpiece 4 has been cut to the previous inner circle. In this case, the path of the cutter 2 in the linear approach mode physically interferes with the workpiece 4 at a point B.
In cases where the workpiece 2 is expected to physically interfere with the workpiece 4 in the conventional inner circle cutting practice, the cutter 2 is moved at a cutting feed rate in the linear approach mode. Therefore, the cutting process time is relatively long.